Quantum networks have been attracting increased interest over the recent years. In such networks, information is carried by quantum systems which are transmitted over transmission links. In the case of transmission of photons over optical transmission links, the transmission distances are limited by absorption and scattering in optical fibers, which result in transmission loss, in the same way as in standard optical networks.
FIG. 1 illustrates a quantum network with direct transmission over an optical fiber. The source node (1) transmits photons to the destination node (2). For application such as quantum cryptography which involves the distribution of quantum keys over the communication link, the transmission of a single photon over a direct transmission link of length greater than 100 km cannot be achieved with an acceptable signal-to-noise ratio (SNR). Indeed, such signal-to-noise ratio (and hence the rate of received photons at the destination node over time) decreases exponentially with the length of the direct transmission link. For example, if we assume a direct transmission link of length L=1000 km, the signal-to-noise ratio is of the order of 10-20. In classical optical networks, transmission losses are reduced through the use of repeater nodes which are placed in between the source node and the destination node, generally separated by a distance which is acceptable from an SNR standpoint given the desired SNR between the source node and the destination node, and which include an amplifier for amplifying a received signal before retransmitting it to the next node of the network. However, the use of such repeaters which provide successive amplification of a signal during transmission over a long-haul communication link (e.g. of length>1000 km) is not efficient in quantum communications, as amplifying a quantum signal implies the loss of the quantum properties of such signal, resulting in the loss of quantum information carried by the signal. This limitation of quantum systems is sometimes referred to as the “no-cloning theorem”.
A so-called “quantum repeater” scheme was proposed in 1998 to allow scalable quantum communications, which aim at extending the span of quantum communications over long distances (H. Briegel, W. Duer, J. L. Cirac & P. Zoller, Phys. Rev. Letter, 81, 5932 (published in 1998)). The use of quantum repeaters (3a, 3b) in this context has led to research carried out to develop so-called quantum memories (4a, 4b), which allow the storage of photons in a storage medium and retrieval of such stored photons.
Photon-echo techniques have been investigated for quantum memory applications. Schemes such as the two-pulse photon echo (2PE) and three-pulse photon echo (3PE), in which an input pulse is followed by one or two strong rephasing pulses, respectively, have been considered. However, the strong optical rephasing pulses used in the 2PE and 3PE techniques invert the atomic population, thereby causing the storage medium to operate in gain regime which generates noise. Controlled reversible inhomogeneous broadening (CRIB) and atomic frequency combs (AFCs) are two different approaches in the field of photon-echo quantum memories which have been proposed to mitigate this drawback. These approaches are unpractical in that they all require a complex preparation phase.
There remains a need for an improved quantum memory method and device which does not require a complex preparation phase.